It has been typical to neglect lateral chromatic aberration in diver's vision in the past. Lateral chromatic aberration occurs with off-axis light between two media. This aberration leads to blurry vision toward the edge of the field of view. Partly this neglect stems from the strongly filtered light visible at significant depths. However, it has also been assumed that the diver will merely turn his or her head to center the viewed object in front of the eyes. While this is expeditious, it does not allow a good field of clear vision. When there is sufficient light across the spectrum, the diver's visual acuity is limited by lateral chromatic aberration. This occurs in recreational dives in shallow, clear water. It also occurs in dives in deeper water where the illumination is from a wide-band artificial source.
Because of the strangeness of the environment, divers are often not aware of the lateral chromatic aberration inherent in their masks. Salvage divers looking for small items on the bottom might pass by an object and miss it merely because it is off-axis in the field. They cannot afford to direct their heads at precisely every minute area of bottom surface. Even though they are sweeping the bottom with their eyes, the view is seriously disturbed off-axis. In the case of a conventional camera lens which has been designed for use in air, optical performance is also seriously disturbed by an unusual change of medium placed before the lens.
An afocal lens system takes a parallel ray of light on the input side and processes them into exiting parallel rays of light having different optical properties. The most common optical property that is modified is angular magnification, but other properties may be selected for optimization. In the cases described here, the lens system is only afocal when operated between the media for which it has been designed.
Monochromatic light in a parallel beam does not undergo focusing or defocusing during passage through a plane piece of glass or transparent plastic when that glass or plastic is used to separate water from air in a diver-mask or an underwater camera-lens protector. Thus, the simple window is afocal. However, the light beam is refracted, according to Snell's Law of Refraction: EQU n.sub.water sin(.crclbar..sub.water)=n.sub.air sin(.crclbar..sub.air)(Equation 1)
where n.sub.water is the index of refraction in water and .crclbar..sub.air is the angle of incidence in water. The other variables are defined similarly. The indices of refraction are generally adjusted so that the index in air is unity. A flat window is not a factor because refraction that occurs at the entrance face is undone at the exit face, with an accumulated refraction of zero.
If the values of the indices of refraction were independent of color, there would be no opportunity for lateral color to be introduced into the view of a camera lens or the field of vision of an observer. However, they are not. A somewhat restricted light spectrum representative of the usable spectrum in shallow, well-lit seawater is depicted in Table 1. The refraction angles are calculated for light incident at an angle 40 degrees from an axis perpendicular to the plane of the lens.
TABLE 1 ______________________________________ Wavelength Refracted Angle in Air (nm) Index of Seawater (40 degrees in seawater) ______________________________________ 1 0.480 1.3438906 59.750 2 0.550 1.3407800 59.523 3 0.620 1.3386056 59.366 ______________________________________
Table 1 demonstrates that eventhough the index of refraction of seawater varies by only about 4 parts in a thousand over this spectral range, there results a 0.38 degree, or 23 arcminute, error. Dispersion is the change of index of refraction verses wavelength. Low-dispersion materials have lowered differences of index of refraction for the same wavelength change than high-dispersion materials. The identity of the material as either a low dispersion or a high dispersion material is a relative concept. For purposes of the present application, low-dispersion refers to a material around the dispersion level of water, and high-dispersion refers to the material with a higher dispersion level than that of water. Normally human vision can resolve two lines separated by 1 to 1.5 arcminutes. Consequently the blurring caused by a 23 arcminute difference is considerable.
By manipulation of Snell's law in equation 1 it can be shown that, to a very good approximation EQU .DELTA..crclbar.=[(n.sub.3 /n.sub.1)-1]tan .crclbar.
where n.sub.1 is the index in water at wavelength 1; and n.sub.3 is the index in the water of a wavelength 3; and, .DELTA..crclbar.=.crclbar..sub.3 -.crclbar..sub.1 is the difference in propagation angles of these two wavelengths in air. The value .crclbar. is the propagation angle in air at a wavelength 2. It can be seen from the aforementioned formula that the difference in refraction .DELTA..crclbar. can be decreased either by turning the head of the viewer so that .crclbar. is minimized or by strongly filtering the light.
Divers have manages to live with this optical problem for years, primarily because of two reasons. The first reason is the strong filtering of deeper water resulting in a quasimonochromatic distribution (toward the blue) of available light. The second reason is that they quickly learn, consciously or unconsciously, to turn their heads or bodies to center the object in the faceplate if they want to look closely at it.
As a consequence of this training and filtering, there has been little attention devoted to lateral chromatic aberration in the design of diver masks. In Human Vision Underwater: Physiology and Physics, by Jo Ann S. Kinney, the only form of chromatic aberration mentioned is longitudinal chromatic aberration (p.80), and then only in the eye. In U.S. Pat. Nos. 5,523,804 and 5,359,371 to Nolan, color is mentioned but only as it relates to the natural filtering of water. In U.S. Pat. No. 5,420,649 to Lewis, lateral color is mentioned, but only as it relates to removing the spectral effects of prisms placed on the inside of a diver's mask to divert light.
In the case of optical windows used to protect cameras designed for use in air, most photographers are willing to restrict the field of vision to achieve reasonable imaging of cameras normally designed for air use when employed underwater looking through the window of a dry box. But there is no physical limitation requiring that this compromise be made. Cameras designed for underwater use usually have special lenses that work immersed in water with minimum lateral color. However, there are special circumstances where such a camera lens may not yet be fitted to a specific advanced imaging system. In such a case, a dry box must be employed and it is necessary to adapt a lens, that normally operates in air, to a water medium. Thus, the situation is not adequately addressed by conventional systems and techniques.